Evaluating the Foraging Ecology of Bigeye and Yellowfin Tuna off the US Continental Shelf.

Austin, Riley¹, Walter J. Golet^1,2, John Logan³, Joe Quattro⁴, ¹School of Marine Sciences, University of Maine, Orono, ME, ²Gulf of Maine Research Institute, Portland, ME, ³Division of Marine Fisheries, Boston, MA, ⁴School of Earth, Ocean, and Environment, University of South Carolina, Columbia, SC

Surveying pelagic environments can be daunting due to the spatial scales which they encompass. Bigeye (Thunnus obesus) and yellowfin (Thunnus albacares) tunas represent two highly mobile, generalist predators which occupy and forage in areas of the epi- and mesopelagic zones. Given the utility of these tunas as biological samplers of their environment and the importance of healthy prey assemblages in this foraging area, diet compositions were investigated. With the objectives of comparing diets between tuna species and assessing spatio-temporal variability and other factors influencing forage in the northwest Atlantic Ocean, stomachs were collected from 199 bigeye and 606 yellowfin tuna captured in commercial longline and recreational rod and reel fisheries from 2018-2020. Stomach contents, identified using genetic barcoding and morphology of whole body, otoliths, and beaks, revealed Illex illecebrosus was the most important prey species of both tunas while prey species primarily found in the mesopelagic and epipelagic constituted the remainder of diets for bigeye and yellowfin tuna, respectively. Time and location of sampling events was the most important factor in differentiating tuna diets which highlights the variability of forage at regional levels and under different oceanographic regimes.